Curtailment Is a Decision, Not an Accident

There is a wind farm off the Angus coast in Scotland called Seagreen. One hundred and fourteen turbines. One point one gigawatts of nameplate capacity. In the year to March 2025, it was paid to not generate for seventy-one percent of its operational hours. Three and a half terawatt-hours of clean electricity, turned away.

The official explanation is congestion. The transmission boundary between Scotland and England — the network engineers call it B6 — cannot carry everything the Scottish wind fleet produces on a windy summer morning. So NESO sends a constraint instruction. Seagreen drops its output. A gas plant in southern England ramps up. The energy gets to London. Just not from the cheapest source.

We call this curtailment. The word is procedural. Technical. Unavoidable-sounding. It is none of those things.

Same physical fact, three different bills

Curtailment is a decision. It is dispatched in milliseconds, by code, in operator control rooms. But the rules that decide who gets curtailed first — and who pays for it — were written by lawyers years before any of these turbines existed. And the rules look very different in different jurisdictions.

In Britain, there is no merit order. Curtailment is what the Balancing Mechanism does. NESO picks the cheapest bid that solves the constraint. The cost — about £1.5 billion of wind constraint payments in 2025 — is socialised through BSUoS, a levy on consumer bills.

In continental Europe, EU Regulation 2019/943 Article 13 says non-market curtailment of renewables should be a last resort, and compensated when it happens. Spain operationalises this explicitly in Annex XV of Royal Decree 413/2014. Germany compensates curtailed renewables through a regulated regime tied to the EEG market premium.

In Texas, there is no statutory renewable priority, no automatic compensation. Curtailment lives inside power purchase agreements. The blunt version, from analyst Matthew Middleton: “The majority of PPAs settle as-produced, which means if a site doesn’t deliver energy to the grid, it doesn’t get paid.”

Four bills. Same physical fact.

The German paradox

The most counter-intuitive finding sits inside Germany’s annual data. The volume of renewable energy being curtailed has stayed roughly flat between 2023 and 2025 — around 9 to 10 terawatt-hours per year. But the compensation bill has fallen, from about €600 million in 2023 to €433 million in 2025.

The mechanism: German compensation is tied to the EEG market premium, which shrinks when wholesale prices rise. So Germany is now curtailing the same amount of clean electricity and paying renewable generators less for the privilege. Meanwhile, negative-price hours rose from 301 to 573 over the same period, and solar curtailment specifically nearly doubled.

The visible political cost is falling. The underlying problem is getting worse.

The silent curtailment nobody is counting

There is a layer of curtailment that does not appear in any annual report from any regulator.

In Germany, the 2025 Solarspitzengesetz imposes a temporary 60% export cap on new sub-100 kW rooftop solar without smart-meter control. Plus zero remuneration during negative-price hours. In California, Rule 21 smart-inverter requirements can throttle export through volt-var control. In Britain, G98 and G99 protection settings trip arrays autonomously when voltage drifts. In Spain, “self-consumption without surplus” requires an anti-injection device.

None of these homeowners would say they were curtailed. None of these megawatt-hours appear in any ledger. But the electrons stop — every day, in firmware the homeowner never sees and could not change if they wanted to. The most curtailed generator on the modern grid may be the one nobody is counting.

What’s actually happening

Curtailment is what happens when there is energy available, somewhere it could go, and the layer in charge of routing it is too slow, too distant, or too uniform, to send it there.

The blade in Angus could be charging a thousand electric cars in Newcastle. The panel in Bavaria could be heating water in the same village. The array in Kern County could be running an electrolyzer twenty kilometres south. None of these absorbers are missing. None of these uses are unprofitable. None of these matches are technically impossible.

What is missing — in 2026 — is a control layer fast enough, local enough, and granular enough, to make the match happen before the only available lever is “switch off the generator.”

Every curtailed megawatt-hour is a coordination failure dressed up as a thermodynamic necessity.

The data gap

There is one more finding worth naming. Germany and Great Britain publish curtailment data with annual line items. Energy curtailed. Compensation paid. Sometimes down to the specific transmission boundary. Spain, the United States, and Sweden do not — at least not at asset level. American curtailment lives inside private power purchase agreements. ERCOT reports congestion outcomes. CAISO reports zonal curtailment.

The most consequential daily decision on the modern electricity grid is, in two of the four largest renewable systems in the world, effectively invisible at the asset level. The fact that we cannot easily count what we are throwing away is itself a finding.

Curtailment is a decision. The only question worth asking is who is making it — and at what speed.

Full transcript available below the audio player.

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